Sphingolipids are thought to play roles i cell-to-cell interaction, regulation of growth, differentiation, and oncogenesis in animals. Recent developments have shown that sphingolipids participate in signal transduction. Despite great progress, much remains to be learned about the specific roles of sphingolipids and their mechanism of action. One limitation in sphingolipid research has been the lack of experimental organisms that are amenable to molecular genetic analysis. During the past grant period our research revealed the potential that Saccharomyces cerevisiae offers for combining molecular genetic analysis with biochemical techniques to studies of sphingolipid synthesis and function. This work has laid the foundation for our proposed Specific Aims in which we will (1) examine the hypothesis that sphingolipid concentrations is maintained in part by regulating the activity f serine palmitoyltransferase, (2) isolate the gene(s) for inositol-P-ceramide (IPC) and ceramide synthetases, overexpress them, and purify and characterized the enzymes. IPC synthetase will be emphasized since it catalyzed a reaction unique to fungi that should be an efficacious target for antifungal drugs, (3) determine the genetic and biochemical basis for the ability of strain 7R4 to bypass the need to make sphingolipids. The unique lipids that accumulate in this strain will be characterized to determine if they are intermediated in the formation of proteins anchored to ceramide and to phosphatidylinositol-glycans, (4) use a genetic strategy to examine the potential role of sphingolipids in signal transduction in S. cerevisiae. The results of our studies should provide a better understanding of sphingolipid synthesis, function(s), and regulation in fungi and other organisms. In addition, they should provide a unique target for designing antifungal drugs needed for treatment of immune compromised humans. As a direct results of our efforts it should be possible to used clone yeast sphingolipid biosynthetic genes to isolate human homologs.